By today's convention, a superhydrophobic (SHPo) surface is defined as a solid surface on which water in air forms a contact angle greater than 150 degrees. Such high contact angles have so far been found only on structured hydrophobic surfaces, the structures typically in micrometers. A typical demonstration of SHPo surfaces is water droplets rolling around on them with little resistance. Recently, SHPo surfaces have shown promise when submerged in water (i.e., no droplet) as well because their surface structures or microfeatures can hold a gas film under water. One compelling application is drag reduction, as the gas held within (in between) the microfeatures lubricates water flows on them, effectively allowing the water to slip on the surface. How slippery a liquid flows on a solid surface is quantitatively defined as slip length, which had never been found to be more than 1 micrometer (too small to be useful) until the advent of SHPo surfaces. Some SHPo surfaces have even demonstrated effective slip lengths in hundreds of micrometers, which are large enough to benefit even regular (i.e., large) fluidic systems. Drag reduction for turbulent flows has also been reported. For drag reduction, SHPo surfaces are considered a superior alternative to the existing bubble injection method because the stable gas upon the surfaces makes the SHPo method passive (i.e., energy efficient) and simple (i.e., easy to implement). Moreover, it has been shown that the minimized solid-liquid contact on SHPo surfaces can resist surface fouling, especially biofouling. Despite its great potential, drag reduction by SHPo surfaces has been considered strictly limited to laboratory conditions because there was no indication that such SHPo surfaces could retain the gas layer long enough under real conditions.
SHPo surfaces are not SHPo anymore once they become wetted (i.e., the liquid enters in between the microfeatures), thereby losing their beneficial properties (e.g., water repellency, drag reduction, biofouling prevention). Because the wetting transition of a SHPo surface inside a liquid is spontaneous, any liquid impregnation, incited by various instigators, has an irrevocable effect against drag reduction. FIG. 1, for example, illustrates the transition of a SHPo surface from a non-wetted state (top) to a wetted state (bottom). For example, the surface may get wet and lose its slip effect if it has defects, the liquid is under pressure, or the gas within the microfeatures diffuses away to the liquid over time. Once the SHPo surface has become wetted as illustrated in FIG. 1, the superhydrophobicity and its attendant benefits in reduction in drag, etc. are lost.
Recently, several approaches have been suggested to increase the stability of the gas layer on a SHPo surface against liquid pressure. For example, a gas layer can be pneumatically pressurized either actively or passively so that it can withstand elevated liquid pressures. Alternatively, hierarchically structures have been employed to make SHPo surfaces more resistant to liquid pressure. These previous improvements do not work unless the liquid pressure is relatively small (e.g., even 0.5 atm is too high). Moreover, these approaches are only preventive measures. They are ineffective once the gas layer is disrupted. The ability to maintain superhydrophobicity under various adverse conditions is needed to utilize SHPo surface in real applications. More desirable for robustness is the ability to restore superhydrophobicity even after the surface becomes wetted by unexpected events. A successful scheme should be able to displace the liquid that has impregnated the surface structures with new gas and restore a stable gas film.